Interest has grown tremendously over the last decade in the use of fats and vegetable oils as a feedstock for manufacturing biodiesel, and many commercial plants have been built. The economic advantage of using vegetable oils as a feedstock, however, can vary significantly based on a number of factors including the fluctuating cost of crude oil, the limited supply of vegetable oils, and competing demands in food applications. Biodiesel is a renewable, alternative diesel fuel consisting of long chain alkyl (methyl or ethyl) esters, made by transesterification of vegetable oils such as those from corn, olive, palm, cottonseed and sunflower seed, or animal fats such as tallow, lard, and butter, as well as commercial products, such as margarines.
Triglycerides, also known as triacylglycerols or TAGs, are the major component of nearly all the commercially important biodiesel feedstock, such as, for example, the fats and oils of animals and plant origin listed above. Triglycerides are formed from a single molecule of glycerol, combined with three fatty acids on each of the glycerol OH groups. The chemical formula of triglycerides is R1COO—CH2CH(—OOC—R2)CH2—OOC—R3, where R1, R2, and R3 are long alkyl or alkenyl chains. The three fatty acids R1COOH, R2COOH and R3COOH can be all different, all the same, or only two the same. Triglycerides with three identical fatty acids (R1═R2═R3) are generally denoted as simple triglycerides. Triglycerides containing more than one type of fatty acid are denoted as mixed triglycerides. Chain lengths of fatty acids in natural triglycerides are variable, but carbon numbers of 16, 18 and 20 carbons are the most common. Triglycerides that include fatty acids that contain only single bonds between carbons along the chain length (alkyl chains) are denoted as saturated triglycerides. Triglycerides containing carbon-carbon double bonds between carbons along the chain length (alkenyl chains) are denoted as unsaturated triglycerides. Unsaturated triglycerides containing a single carbon-carbon double bond are denoted as monounsaturated triglycerides, while those containing two or more carbon-carbon double bonds are denoted as polyunsaturated triglycerides.
Biodiesel is produced by transesterification of triglycerides. A representative transesterification reaction that produces biodiesel in the form of methyl esters from a representative triglyceride biodiesel feedstock containing R1, R2, and R3 fatty acids, is illustrated in FIG. 1. In the transesterification reaction, the triglyceride is reacted with alcohol, such as, for example methanol in FIG. 1, in the presence of a catalyst, typically a strong alkali such as, for example, sodium hydroxide or potassium hydroxide. The reaction can also be acid-catalyzed or enzymatic. The reaction products are glycerol and the methyl esters of the R1, R2, and R3 fatty acids. The methyl esters can then be used as biodiesel fuel.
Most natural fats and oils are complex mixtures of many different triglycerides. The exact triglyceride composition of a fat or oil further varies with the source and growth conditions of the feedstock. Significant research has been focused on the development of new non-food vegetable oil sources as a sustainable feedstock for biodiesel manufacturing. For example, certain species of algae that contain high amounts of oil and have high growth rates are considered a promising potential feedstock for next generation biofuels. New feedstock, combined with new process technologies and optimized production plants can help to alleviate some of the cost pressures and favor the trend toward bio-chemical alternatives.
Process modeling and simulation technology has become an established practice for rapid process development and optimization in the chemical and petrochemical industry. Such technology can also play a key contributing role in the development and optimization of the process technologies and process plants for biodiesel production. One of the challenges limiting the use of process modeling and simulation technology in biodiesel processes is the lack of proven models and databanks for estimating the thermophysical properties of vegetable oils, blends, and, most importantly, the individual triglyceride components that make up the oils. Accurate estimation of the thermophysical properties, such as, for example, vapor pressure, enthalpy of vaporization, liquid heat capacity and enthalpy of formation, liquid molar volume and viscosity, is an essential first step to developing flowsheet models for design, optimization and control of biodiesel production processes. See Myint, L. L., and El-Halwagi, M. M., “Process analysis and optimization of biodiesel production from soybean oil,” Clean Techn. Environ. Policy, DOI 10.1007/s/0098-008-0156-5 (June, 2008).
There is a limited amount of available information for estimation of thermophysical properties for triglycerides. Most of the data is based on the traditional functional group approach. For example, Ceriani and Meirelles reported a group contribution method for the estimation of the vapor pressure of fatty compounds and the optimized parameters. See Ceriani, R., Meirelles, A. J. A., “Predicting Vapor-liquid Equilibria of Fatty Systems,” Fluid Phase Equilibria, 215, 227-236 (2004). All the fatty compounds gathered in the experimental data bank were split into eight functional groups: CH3, CH2, COOH, CH=cis, CH=trans, OH, COO, and CH2—CH—CH2. The same authors later extended this functional group approach to predict the viscosity of triglycerides. See Ceriani, R., Goncalves, C. B., Rabelo, J., Caruso, M., Cunha, A. C. C., Cavaleri, F. W., Batista, E. A. C., Meirelles, A. J. A., “Group Contribution Model for Predicting Viscosity of Fatty Compounds,” Journal of Chemical and Engineering Data, 52, 965-972 (2007). Separately, a rather cumbersome group contribution method was developed to predict the melting points and the enthalpies of fusion of saturated triglycerides. See Zéberg-Mikkelsen, C. K., Stenby, E. H., “Predicting the Melting Points and the Enthalpies of Fusion of Saturated Triglycerides by a Group Contribution Method,” Fluid Phase Equilibria, 62, 7-17 (1999). Although this approach can be used to identify a unique set of functional groups and parameters to match available experimental data for triglycerides, the functional group approach is too simplistic to model the variations in thermophysical properties of various triglycerides.
Similar difficulties are encountered in the estimation of thermophysical properties for mono- and diglycerides. Vegetable oils comprise 90-98% triglycerides and small amounts of mono- and diglycerides. Monoglycerides (monoacylglycerols or MAGs) are fatty acid monoesters of glycerol and exist in two isomeric forms, 1-monoglycerides and 2-monoglycerides, depending on the position of the ester bond on the glycerol group. Diglycerides (diacylglycerols or DAGs) consist of two fatty acid chains bonded to a glycerol molecule by ester linkages. They are typically found as 1,2-diglycerides and 1,3-diglycerides. Mono- and diglycerides are also formed as intermediates in the transesterification of triglycerides, which is believed to proceed as the three consecutive and reversible reactions shown in Eqs. 1-3:TAG+ROHDAG+R′COOR  (1)DAG+ROHMAG+R′COOR  (2)MAG+ROHGlycerol+R′COOR  (3)see Freedman, B., Butterfield, R. O., Pryde, E. H., Transesterification Kinetics of Soybean Oil, Journal of the American Oil Chemists' Society, 63, 1375-1380 (1986).
Due to the importance of mono-, di- and triglycerides for the production of biodiesel, a new approach is needed for accurate and systematic correlation and estimation of the thermophysical properties of individual mono-, di-, and triglyceride components, and of the mixture properties of fats and oils in biodiesel feedstock.